This invention relates to cathode ray tubes having image storage capabilities. Two conventional types of cathode ray tube memories or image storage devices are the memoscope, a memory device utilizing electrostatic charge as the means of retaining information, and the dark trace tube, wherein the face plate of a cathode ray tube is provided with a layer of "dark trace" material such as an alkali halide compound.
Storage tubes based on the memoscope principle consist of a complicated apparatus which is difficult to manufacture and which is restricted in size to about 7 inches in diameter. The contrast in these tubes is not satisfactory, and brightness is quite limited. Furthermore, overlay apparatus used in conjunction with this type of tube is very complicated.
Dark trace storage tubes, such as that described in U.S. Pat. No. 3,148,281 granted to N. F. Fyler on Sept. 8, 1964, are also difficult to manufacture. A monocrystalline layer of a dark trace material such as potassium chloride must be formed on the inner surface of the cathode ray tube face plate, and a layer of phosphor is deposited over the monocrystalline layer. It is difficult to form such tubes in sizes greater than 7 inches in diameter. A serious disadvantage of this type of storage tube is that outgassing from the potassium chloride can adversely affect the cathode of the tube and thereby shorten its life. Furthermore, the opaque areas which form in the potassium chloride layer to increase the optical density thereof are highly susceptible to heat with the result that the image readily fades at room temperature.